The use of thyristors or triacs as speed controls for electric motors is well-known in the art. Assignee of the instant application is also assignee of U.S. Application Ser. No. 915,977, filed June 16, 1978, by Cockroft et al. While the combination of a mechanical governor and triac providing phase angle speed control has been adequate to supply the needs of most home appliance users for motor speed accuracy, applicant has found that a higher degree of motor speed accuracy is demanded by the purchaser of a relatively expensive appliance, such as that disclosed in U.S. Pat. No. 4,176,971, which is assigned to the assignee of the instant application.
One of the problems encountered with the mechanical governor-thyristor speed control is mistriggering of the thyristor. Since the rotational speed of a universal series induction electric motor is a function of the amount of power supplied to rotate a selected load, the average power supplied to a motor can be controlled by switching a triac connected in series with the motor into conduction at a preselected phase angle, as is well-known in the prior art. Some problems, however, have been encountered when small amounts of power are to be supplied to the motor. This necessitates triggering the triac a relatively short time before the next AC zero crossing or at a relatively small conduction angle. At times, it has been found that triggering at very small conduction angles can cause the triac to conduct for larger conductions angles of the next several half cycles than would be required to maintain speed. This is because the response time to motor speed changes is relatively long due to the inertia of the mechanical components of the governor in the prior art systems such as the one disclosed in Application Ser. No. 915,977, previously mentioned. The unwanted additional power thereby supplied has a tendency to make the motor race or vary in speed when the motor should be rotating at the preselected slower speed.
In addition, it is desirable to provide feedback to the triac or motor controller through nonmoving or electronic elements which will not wear.
What is needed is a closed loop motor speed control system having highly accurate and rapid response to motor speed changes.